1. Modem
Eng. M. Zamanian
By Amir Taherin

2. Contents 1
Definition 2
History 3
V.21, V.22, V.22bis 4
Modulation

3. Contents 5 Amplitude shift keying (ASK) 6 Frequency shift keying (FSK)7
Phase shift keying (PSK) 8
More Complex Modulation

4. Contents 9 Advanced Frequency Shift Keying Advanced Phase Shift Keying10
Advanced Phase Shift Keying 11
Quadrature Phase Shift Keying 12
Quadrature Amplitude Modulation

5. Contents QAM Applied To Dialup Modem Protocols Modem and NGN13
QAM Applied To Dialup 14
Modem Protocols 15
Modem and NGN 16
Preferences

6. Definition
Modem
Modulation
Demodulation
Mod
Dem

7. History Hayes, 103A 300-bit/s Data-Phone 201A, 201B
IBM, Punched card, 25 bit/s
George Stibitz
1982
1943
1940

8. V.21, V.22, V.22bis In Bell telephony Answarin modem Send 0  1070 Hz
These frequencies were chosen carefully
Suffer minimum distortion on the phone system
Not harmonics of each other

9. Modulation

10. Amplitude shift keying (ASK)
Use different amplitude to represent 0 and 1.
Simple, low bandwidth
Sensitive to interference.
Multi-path propagation, noise or path loss heavily influence the amplitude.
A constant amplitude in wireless environment can not be guaranteed.
Used in wired optical communication.
A light pulse =1, no light =0.

11. ASK Sample

12. Frequency shift keying (FSK)
Binary FSK (BFSK)
One frequency for 0 and one frequency for 1.
Needs larger bandwidth
Avoid discontinuity
Discontinuity creates high frequencies as side effects.
Demodulation
Use two band-pass filters for 2 frequencies.

13. FSK Sample

14. Phase shift keying (PSK)
Use shift in phase to represent data.
Binary PSK (BPSK)
Shift the phase by 180.
Synchronization is important
More resistant to interference
More complex transmitters and receivers.

15. PSK Sample

16. More Complex Modulation
Low efficiency in ASK, FSK, PSK.
High bandwidth

17. Advanced Frequency Shift Keying

18. Advanced phase shift keying
Phase domain: use a vector (or a point) in the plane to represent the signal.
Length of the vector: amplitude
Angle: phase

19. Advanced phase shift keying
BPSK (Binary Phase Shift Keying)
bit value 0: sine wave
bit value 1: inverted sine wave
very simple PSK
robust, used in satellite systems

20. Quadrature Phase shift Keying
QPSK (Quadrature Phase Shift Keying)
2 bits coded as one symbol
Symbol determines shift of sine wave
Needs less bandwidth compared to BPSK
More Complex
Transmitter and receiver are synchronized very often.

21. Quadrature Phase shift Keying

22. Quadrature Amplitude Modulation
Quadrature Amplitude Modulation(QAM): combines amplitude and phase modulation
It is possible to code n bits using one symbol
2n discrete levels, n=2 identical to QPSK
Bit error rate increases with n, but less errors compared to comparable PSK schemes

23. Quadrature Amplitude Modulation

24. QAM Applied To Dialup
QAM is also used with dialup modems as a way to maximize the rate at which data can be sent
Most telephone connections transfer frequencies between 300 and 3000 Hz

25. QAM for V.32 Dialup Modems

26. QAM for V.32bis Dialup Modems

27. Modem Protocols

28. Modem Protocols ITU Standard Name V.1 – V.8bis General Standards
Interfaces and voice-band modems
V.35
Wideband Modems
V.41 – V.44
Error Control and Data Comparison
V.61 – V.92
Simultaneous transmission of data and other signals
V.110 – V.120 – V.150.1
Interworking with other networks
V.230
Interface layer specifications for data communication
Applies to V.250–V.299
Control procedures
V.300
Digital circuit modems

29. Modem Protocols V.34 V.42bis V.90 V.34+ Speed up to 33.6 Kbit/s
For Error Control
BTLZ algorithm (British Telecom Lempel Ziv)
Alan Clark
V.90
56 Kbit/s Download
33.6 Kbit/s Upload
Known as V. last

30. Media Gateway Controler
Modems and NGN
Service
Media Gateway Controler
Packetized Network
Access

31. Refrences
ITU-T ( ). "Data communication over the telephone network". ITU. Retrieved
V.22, V.22bis & V.32 handshakes
ITU-T Recommendations: V Series
Modem

32. Question ? ? ? ? ? ? ?

33. Thank You !